Surface-enhanced Raman spectroscopy (SERS) has been used as an analytical tool to observe trace amounts of chemical and biological molecules due to its capability of giving real-time molecular vibrational information under ambient conditions. It has the advantages of using extremely small amounts of analyte without amplification or manipulation of the samples, and the extremely short time frame for acquisition of the spectra. SERS requires minimal sample preparation and is non-destructive to the sample, which allows real time analysis and great potential for multi-component analysis. The well-known Raman enhancement effect of SERS arises from either the adsorption or close proximity of an analyte to a metal substrate. The morphology of the metallic structure plays a major role in determining the magnitude of signal enhancement and sensitivity of detection. Early SERS substrates included a random distribution of roughness features produced by oxidation reduction on a metal electrode or evaporation of thin metal film on a flat substrate. Various forms of nanostructure have been explored to enhance SERS effects, for example; rough metallic surfaces by chemical etching, silver films on TiO2, colloidal silver nanoparticles, silver nanoparticle arrays fabricated by nanosphere lithography, electro-deposition of silver on silver films at high potential, aligned monolayer of silver nanowires. However, many of these methods are either expensive or time consuming, and it is not easy to make reproducible substrates of the correct surface morphology to provide maximum SERS enhancements. Without uniformity and good reproducibility of the metal substrates, the attainment of reproducible spectra remains a major challenge for SERS.
Despite its extremely high and unique sensitivity, the application of SERS has not been incorporated into the development of practical in-situ analytical tool for real-time sensing or detecting. Usually SERS measurements are carried out by conventional Raman scattering spectrometer, which is bulky, expensive, and not easily accessible. Therefore, there is a need in the art to overcome these limitations.